The inventive concepts described herein are generally related to a power amplifier, and more particularly, to a power amplifier having improved efficiency when operating in a back-off power region.
There is an increasing demand in the industry to provide wireless communication systems capable of achieving high data transfer rates with limited frequency resources. Efforts have been thus been made to optimize modulation schemes used in wireless communication systems to meet such demand. The optimization of modulation schemes has been intended to achieve an increase in the peak to average power ratio (PAPR) of signals transmitted by wireless communication systems.
In order to maintain good linearity, power amplifiers transmitting signals with a high PAPR inevitably operate in a back-off power region more frequently than when transmitting signals with low PAPR. However, frequent operation in the back-off power region may reduce operating efficiency of power amplifiers. Accordingly, there has arisen a demand for a power amplifier that operates with high efficiency in the back-off power region while maintaining good linearity. To meet this demand, research has focused on supply voltage modulated power amplifiers.
Typically, supply voltage modulated power amplifiers are configured to include a radio frequency (RF) power amplification unit that amplifies an RF input signal and outputs an amplified signal, and a supply voltage modulation unit that modifies a bias voltage supplied to the RF power amplification unit based on amplitude of the RF input signal. Since the supplied bias voltage can be reduced when the amplitude of the RF input signal is small, supply voltage modulated power amplifiers may be more efficient than power amplifiers that apply a fixed-value bias voltage to corresponding RF power amplification units. Moreover, since the linearity of the supply voltage modulated power amplifiers are mainly influenced by the supply voltage modulation unit and not by an RF power amplification unit, the use of supply voltage modulated power amplifiers help to simplify and enable design of power amplifiers having high efficiency.
FIG. 4 is a schematic view illustrating the configuration of a conventional supply voltage modulated power amplifier. Supply voltage modulated power amplifier 100 includes an RF power amplification unit 110 that amplifies an RF input signal, and a supply voltage modulation unit 120 that modulates a bias voltage supplied to the RF power amplification unit 110 based on an envelope signal obtained responsive to the peak values of the RF input signal. Supply voltage modulation unit 120 includes a DC-to-DC converter that converts the bias voltage.
However, in the case of using a DC-to-DC converter in supply voltage modulation unit 120, the efficiency of RF power amplifier unit 110 of supply voltage modulated power amplifier 100 may decrease depending on the bandwidth of the RF input signal. Normally, the overall efficiency of supply voltage modulated power amplifier 100 may be defined as (efficiency of RF power amplification unit 110)×(efficiency of supply voltage modulation unit 120). As the bandwidth of the RF input signal increases, the efficiency of supply voltage modulation unit 120 including the DC-to-DC converter decreases, resulting in a decrease in the overall efficiency of supply voltage modulated power amplifier 100. Consequently, when supply voltage modulated power amplifier 100 is used in a wireless communication system with a broad band envelope signal, such as Wideband-Code Division Multiple Access (WCDMA) or Long Term Evolution (LTE) systems, the level of improvement in overall efficiency tends to be limited due to a reduction in the efficiency of the DC-to-DC converter.
Additionally, when the DC-to-DC converter within supply voltage modulation unit 120 is switched on and off, switching noise may appear in the output of supply voltage modulated power amplifier 100, adversely influencing operation of the wireless communication system.
Furthermore, the DC-to-DC converter within supply voltage modulation unit 120 typically includes an inductor having an inductance ranging from hundreds of nH to a few uH. It can be difficult to integrate the inductor into a single chip due to the size of the inductor. Moreover, when a low-loss inductor which is relatively large and thick is used to improve efficiency, an increase in the size of the entire circuit may be concomitantly required. These factors contribute to difficulties in developing wireless communication systems of desired compact size.
Accordingly, there has been a need to develop a supply voltage modulated power amplifier capable of overcoming the deficiencies present in conventional power amplifiers employing a DC-to-DC converter, and to provide a power amplifier capable of achieving high operating efficiency in a back-off power region.